The invention relates to a needle printing head, and more particularly, to a printing head in which selected printing needles can be prevented from printing by friction brakes.
In known prior art wire or needle printers, a printing mechanism is used whereby each symbol is composed of several individually printed points wherein the printing is done in a point raster style. Each single printed point is produced by a printing element which must be selected and made to print according to the element's position within the symbol raster. For this purpose, appropriately designed drive devices may be used, which devices are controlled by means of electronic circuitry.
German patent application 24 58 809 shows a printing unit in which control of the individual printing elements of the print mechanism is effected through the use of electromagnetic transducers. Braking means are provided for each of the printing elements and the said means are individually controllable so that selected patterns of the same may be used to lock selected printing elements in a given position in the mechanism. The various means includes mechanical-type locks, magnetic element locks and electrostatic element locks. Printing is accomplished by movement of a rotary drum towards the printing elements wherein a recording medium is trapped therebetween. Those printing elements which have been locked in place by the appropriate means bear on the recording medium in order to effect the printing thereon, while those elements which have not been locked in place are displaced by the movable drum and do not bear with sufficient pressure upon the recording medium to cause a point to be printed. The movement of the backing drum toward the printing needles is one which is effected only through the use of a great amount of power, and necessarily cannot be done with sufficient rapidity in order to achieve satisfactory results.
U.S. Pat. No. 3,715,020 issued Feb. 6, 1973 to Nordin discloses a wire element printing mechanism in which a vertical array of printing elements is selectively braked in order to produce a desired pattern. In the Nordin patent, each printing wire is disposed in a semi-rigid sleeve, and an array of brakes are arranged so as to be in a position to selectively bear on one end of each of the sleeves. When a given sleeve is clamped by means of the brake, a force on one end of the printing wire will cause the other end thereof to protrude from the sleeve and to produce a print mark. When the sleeve is not restrained by means of the brake, the same force applied to one end of the print wire results in a displacement of the wire and the sleeve from its normal rest position, but the wire is not caused to further protrude from the other sleeve end and no printing takes place.
There is a need in the prior art for a printing mechanism in which a plurality of printing wires are arranged in a matrix array and in which individual brakes are provided for each of the printing wires so that upon receipt of a printing force by the wire, the condition of the brakes will be determinative of whether or not a print mark will be made to appear on a recording medium.